I am a radiologist— used to do voiding cystourethrograms (VCUGs) in residency routinely, except I had to place the catheter myself. Hated doing them. Anyhow, this child’s problem is an incompetent valve from the urethra into the urinary bladder, so urine is flowing upstream back into the kidney. The dilation of the renal calyces means it’s a lot. This is the reason the UTIs are recurrent. A lot of the comments here are addressing the UTIs, but that is not the problem, but a result of the vesicoureteral reflux.

There is a good comment below by u/rayshade with more information.

I am a radiologist— used to do voiding cystourethrograms (VCUGs) in residency routinely, except I had to place the catheter myself. Hated doing them. Anyhow, this child’s problem is an incompetent valve from the urethra into the urinary bladder, so urine is flowing upstream back into the kidney. The dilation of the renal calyces means it’s a lot. This is the reason the UTIs are recurrent. A lot of the comments here are addressing the UTIs, but that is not the problem, but result of a the vesicoureteral reflux.

There is a good comment below by u/rayshade with more information.